How to create an orthosis for a .stl hand?
The workflow in this case would go like this:


In edit mode, select appropriate faces.
Shift+D to duplicate faces.
P>Selected to separate selected faces, then Tab to enter object mode.
Select new object, created from the thumb faces and add Shirnkwrap modifier.Use eyedropper to set hand as a Target for the modifier.
From there you can give the new object shape of orthosis and add Solidify modifier to give some thickness to the model.Somewhat like this: